The present invention relates in general to the inspection of check valves and in particular to a new and useful system and method of inspecting a check valve through inductive motion sensing. Maintaining reliable operation of check valves in power plants and other industries employing these critical valves requires periodic inspection to insure the valves are operating properly. One method provides for disassembling the check valve and visually examining and inspecting all parts for potential wear and tear. If any worn or defective parts are found they are replaced and the valve reassembled and returned to service. When using this method, it is very common to assemble a valve that appears to be in good condition after disassembling and inspection, however, proves to be ineffective upon reassembly and return to service.
A more cost effective method for inspecting these valves is to verify the valve function without having to disassemble each valve. For those valves that are inspected without disassembling there are several aspects which enter into the functional verification of each valve. One major aspect is the verification of the valve DISC motion.
One technique in verifying functions through valve DISC motion is to use an ultrasonic transducer in order to send a sound pulse through the casing of the valve for reflection off of the DISC. As the DISC moves within the valve, the distance that the sound pulse travels between the casing and the DISC changes. Because of this change, the time delay between the initial pulse from the ultrasonic transducer and the received reflection from the valve DISC is affected. This problem requires that valve be filled or at least partially filled with water or another sound conducting medium for proper inspection. The ultrasonic transducer technique, however, cannot be used for those check valves that are air-filled or gas-filled.
Electromagnetic methods have been used in order to inspect and test check valves without filling with water or a sound conducting fluid. Also, this method has been used on those check valves which otherwise do not support acoustic detection of valve DISC motion. If the valve DISC is made of carbon steel or a ferromagnetic material, it is possible to place a DC magnet of relative strength on one side of the valve and sense fluctuations resulting from the DISC passing through the magnetic field on the other side of the valve using Hall-effect sensors. This method has proved successful for valve casings made of ferromagnetic material and for DISC made of ferromagnetic material. Some sensitivity is also possible for non-ferromagnetic DISC materials.
Another approach for inspecting check valves is to use a pair or a set of coils that are either located on opposite sides of the outside of the valve casing, or in close proximity to one another. This coil arrangement, however, must insure that the coil fields are influenced by the location of the flapper. One of the coils on the valve casing is excited with an AC or sinusoidal excitation. This excitation is typically several volts at 100 to 10,000 hertz. The excitation induces an AC magnetic field in the check valve which coil. Small fluctuations in the current of the second coil are influenced by the motion of the flapper within the magnetic field. These small fluctuations may be amplified to sense the motion of the valve DISC.
A major drawback found in using coils for sensing motion of a valve flapper is that the inductive coil sensor is of simple construction and requires a very large coil in order to produce any significant voltage amplitudes. In some cases, the coil must be wrapped around the valve which can be very difficult to operate in most test environments. Because the voltage and current demands for achieving good sensitivity are very high, large and heavy power amplifiers must be used. Thus, an actual valve test proves to be very cumbersome and costly.